Japanese Patent Early Publication No. 3-156897 discloses a prior art electronic ballast for a high intensity discharge lamp with a dimming capability. The prior art ballast includes a power converter which converts a DC voltage into a low frequency rectangular waveform AC voltage being applied to the discharge lamp, a controller which controls the power converter, and a dimmer which provides a dimmer command of varying a dimming ratio, i.e., a percentage of actual or reduced lumens to the rated lumens. In response to the dimmer command, the controller controls the power converter to vary a lamp power being supplied to the discharge lamp for reducing the lamp power in accordance with the dimming ratio. In order to keep the color temperature of the lamp within a tolerable range while varying the lumens, the power converter is controlled to generate high frequency ripples superimposed on the low frequency rectangular waveform AC voltage, as shown in FIG. 1. That is, when the dimming is requested, the controller operates to increase an amplitude of the high frequency ripples relative to that of the low frequency rectangular waveform AC voltage, thereby lowering the lamp power being supplied to the discharge lamp while reducing a deviation of color temperature of the lamp. However, it is found that this control scheme is satisfactory only to a limited dimming extent of about 80% of the rated lumens, because of that the amplitude of the ripples is only allowed to increase up to about 70% relative to the amplitude of the low frequency wave form AC voltage.